Processes to recover zinc and lead from naturally occurring ores containing both of those metals have long been established. A typical process, known as the "Imperial Smelting Process" (ISP), comprises three major steps. The first step is a travelling grate sintering step used to convert mixed sulfide ores containing zinc sulfide (ZnS), lead sulfide (PbS) and cadmium sulfide (CdS) to oxides. In the second step, the oxide sinter is mixed with coke and fluxes and treated in a short shaft blast furnace to produce a slag and top gas containing zinc, lead, cadmium and carbon monoxide gases.
In the third step, the metallic vapors contained in the top gas are recovered from a gaseous stream in a splash condenser. It is the condensing step that is of interest in the present application, as the invention is directed toward a condensing process and apparatus.
In the typical condensing process, top gases are passed from the blast furnace to the condenser at a temperature of at least 1100.degree. C., which is about 200.degree. C. above the boiling point of zinc (907.degree. C.). The condenser consists of a chamber partially filled with molten zinc, into which are immersed one or more impeller type splashes. The impellers throw a storm of zinc droplets into the gas stream as it passes through the condensing chamber. The molten zinc bath is maintained at a temperature of about 500.degree. C. Because of the temperature differential between the zinc bath and the gaseous stream, the droplets act to shock-cool the gas stream to a temperature very near that of the zinc bath, causing the zinc vapor in the gas stream to condense to a liquid phase. The liquid zinc then falls to the zinc bath, which gradually increases in volume and is tapped to produce marketable product forms.
The efficacy of the molten zinc condensing process is limited by the physical and thermochemical properties of zinc. The first limitation is imposed by the vapor pressure of zinc. The element zinc has a very narrow range between its melting and boiling points (419.degree. and 907.degree. C., respectively). For this reason, zinc has a significant vapor pressure at any temperature over the melting point. This feature negatively impacts the ability of a zinc splash condenser to condense zinc from the gas stream.
To illustrate the vapor pressure limitation, suppose a gas stream enters the condenser at 1100.degree. C. and contains 20% vapor by volume. The zinc pressure in the gas stream is therefore 0.2 atmospheres. The action of the zinc splash in the condenser cools the gas stream to about 700.degree. C. (approximately 200.degree. C. under the zinc boiling point). Though it might be expected that the zinc gas would condense and become liquid at 700.degree. C., this does not occur because the zinc vapor pressure existing over a bath of molten zinc at 700.degree. C. is 0.65 atmospheres. Because the incoming gas stream contains only 0.2 atmospheres of zinc, the zinc condensing medium would vaporize and leave the system, resulting in a net loss of condensed zinc, rather than the desired net gain.
The limitations imposed by vapor pressure considerations dictate that, at equilibrium, the condensation of zinc from a 20% zinc gas stream will not reach 90% efficiency until the operating temperature of the condenser is lowered below 500.degree. C. For this reason, molten zinc splash condensers are routinely operated at below 500.degree. C.
The necessity of operating a molten zinc splash condenser at below 500.degree. C. introduces a second limitation on the condensing process, relating to the reduction of the metal oxides formed in the blast furnace or smelter. The zinc-bearing gases destined for treatment in the splash condenser are generated by the carbothermic reduction of zinc oxides, according to the following reaction: EQU ZnO+C.fwdarw.Zn+CO (1)
Although Reaction 1 is the significant reaction in the zinc recovery process, taking place at elevated temperatures, other reactions are possible under such conditions: EQU ZnO+CO.revreaction.Zn+CO.sub.2 ( 2) EQU C+CO.sub.2 .revreaction.CO (3)
The latter two reactions are reversible and can proceed in either direction, depending on operating conditions. However, because of Reaction 2, and others such as: EQU FeO+CO.fwdarw.Fe+CO.sub.2 ( 4)
and EQU PbO+CO.fwdarw.Pb+CO.sub.2 ( 5)
some carbon dioxide will always be present in the reaction system. The presence of carbon dioxide enables zinc oxide (ZnO) to be formed by the back reaction of equation (2). This back reaction clearly defeats the purpose of the entire operation, the objective of which is to form reduced zinc. Thus, the Imperial smelting process, and other condensing processes of a similar nature teach toward operating modes which minimize the impact of the back reaction. But because the back reaction is favored by decreased temperature, its negative impact is increased as the condenser operating temperature is lowered. Thus, operating temperatures of under 500.degree. C., which are necessary to minimize zinc vapor pressure limitations, will favor the back reaction, thus decreasing the yield of reduced zinc. For example, the possible recovery of zinc from a gaseous stream containing 20% zinc by volume is about 85% at 500.degree. C. based on vapor pressure considerations. However, at this temperature the back reaction (2) is so strongly favored that 30% of the zinc is lost as oxide.
To minimize the problems caused by vapor pressure and thermochemical factors, the following condenser operation procedures have generally been followed. First, the incoming gas stream is made as rich in zinc vapor as possible, and contains CO and CO.sub.2 in a ratio of at least 100:1. Second, the zinc condenser is operated at 480.degree. C. Third, contact between the zinc splash and the incoming gas stream is maximized to cool the gas stream to 480.degree. C. as rapidly as possible, which limits the time during which the back reaction can occur.
Even if the above-identified condenser operation procedures are employed, the thermodynamic and chemical limitations of the process significantly restrict the efficiency with which zinc may be recovered from a gas stream. Clearly, a method is needed for increasing the efficiency of zinc recovery.
The typical zinc condensing system could be vastly improved in efficiency by any modification that could: (1) enable operation of a condenser system at a higher temperature, thereby restricting the back reaction; or (2) restrict the back reaction at the lower operating temperature by some other means. Neither such method is currently available for use in a molten zinc-type splash condenser.